1. Field
Embodiments of the present invention generally relate to a method for improving critical dimension (CD) microloading in plasma etching a mask layer and, more specifically, to a method for etching a mask layer (e.g., an absorber layer) for both phase shift and binary photomask fabrication and EUV photomask fabrication.
2. Description of the Related Art
In the manufacture of integrated circuits (IC), or chips, patterns representing different layers of the chip are created by a chip designer. A series of reusable masks, or photomasks, are created from these patterns in order to transfer the design of each chip layer onto a semiconductor substrate during the manufacturing process. Mask pattern generation systems use precision lasers or electron beams to image the design of each layer of the chip onto a respective mask. The masks are then used much like photographic negatives to transfer the circuit patterns for each layer onto a semiconductor substrate. These layers are built up using a sequence of processes and translate into the tiny transistors and electrical circuits that comprise each completed chip. Thus, any defects in the mask may be transferred to the chip, potentially adversely affecting performance. Defects that are severe enough may render the mask completely useless. Typically, a set of 15 to 30 masks is used to construct a chip and can be used repeatedly.
A photomask is typically a glass or a quartz substrate giving a film stack having multiple layers, including an absorber layer, capping layer and a photomask shift mask layer disposed thereon. When manufacturing the photomask layer, a photoresist layer is typically disposed on the film stack to facilitate transferring features into the film stack during the subsequently patterning processes. During the patterning process, the circuit design is written onto the photomask by exposing portions of the photoresist to extreme ultraviolet light or ultraviolet light, making the exposed portions soluble in a developing solution. The soluble portion of the resist is then removed, allowing the exposed underlying film stack being etched. The etch process removes the film stack from the photomask at locations where the resist was removed, i.e., the exposed film stack is removed.
With the shrink of critical dimensions (CD), present optical lithography is approaching a technological limit at the 45 nanometer (nm) technology node with small features. Next generation lithography (NGL) is expected to replace the conventional optical lithography method, for example, in the 32 nm technology node and beyond. There are several NGL candidates, such as extreme ultraviolet (EUV) lithography (EUVL), electron projection lithography (EPL), ion projection lithography (IPL), nano-imprint, and X-ray lithography. Among these, EUVL is the most likely successor due to the fact that EUVL has most of the properties of optical lithography, which is more mature technology as compared with other NGL methods.
One of the problems in patterning features with small dimension features is the occurrence of a microloading effect, which is a measure of the variation in etch dimensions between regions of high and low feature density. The low feature density regions (e.g., isolated regions) receive more reactive etchants per unit surface area compared to the high feature density regions (e.g., dense regions) due to larger total expose of surface area in the dense regions, thereby resulting in a higher etching rate in the low density regions. The sidewall passivation generated from the etch by-products exhibited the similar pattern density dependence where more passivation is formed for the isolated features due to more by-products being generated in the low feature density region. The difference in reactive etchants and the passivation per surface area between these two regions increase as feature density difference increase. Thus, due to different etch rates and by-products formation in high and low feature density regions, it is often observed that while the low feature density regions have been etched and defined in a certain desired and controlled vertical dimension, the high feature density regions are bowed and/or undercut by the lateral attacking due to the insufficient sidewall passivation or insufficient etching selectivity of the adjacent layers disposed in the film stack to sustain the film stack until completion of the etching process. In many cases, the low feature density regions are often etched at a faster rate than the high feature density regions, resulting in a deformation, line edge roughness or tapered top portion of the etched layer in the low feature density regions. Insufficient selectivity among the material layers disposed in the film stack in high and low feature density regions often results in inability to hold critical dimension of the etch features and poor patterned transfer.
Thus, there is a need for an improved etch process for etching an absorber layer in a film stack utilized to form a photomask with high etching selectivity.